1. Field of the Invention
This invention relates to a motor-driven camera of the prewind type in which all frame portions of film are preliminarily wound onto a take-up spool and then the film is rewound for every photographing shot.
2. Description of the Related Art
The so-called automatic film loading by which blank film winding is performed when a camera is loaded with a roll of film is intended to ensure that the leader part of the film is reliably wound onto a spool of the camera. However, the automatic film loading would fail if the position of the film perforation does not coincide with the teeth of a sprocket or if the leader part is not placed in a designated position. It has been practiced to wind several frame portion film by using a motor or the like and, at the same time, to detect a movement of the film. Then, if no movement is detected, the automatic film loading is considered to have failed.
However, in the case of a camera of the prewind type, the film loading process for winding a part of the film onto the spool of the camera is immediately followed by a continuous film winding process for winding the film onto the spool to the end part of the film. In accordance with the prewind system, therefore, it is impossible to find the entire length of the film until completion of the prewinding action. The lengths of the commercially available film products are limited to several different lengths. In some cases, however, the photographer wishes to cut the film to a desired length before use. The camera should be arranged to be capable of handling such a film length. If the automatic film loading is determined to have failed by detecting the movement of the film in accordance with the conventional method in such a case, the automatic film loading would be determined to have failed, despite the performance of the loading, as the film comes to a stop also upon completion of the prewinding action.
For a camera having a prewind system, a method for making a discrimination between the failure of the automatic film loading and the completion of the preliminary film winding has been disclosed in U.S. Pat. No. 4,460,256. This patent is for a motor-driven camera. The camera has a prewind system and is arranged to have all the frame portions of film first wound in response to closing of the back lid of the camera; and, after that, each of the frame portions of film is rewound back into a film cartridge at the end of every photographing shot. More specifically, the camera is provided with a mechanical film counter mechanism. The rotation of a sprocket is used as a drive source for rotating a disc-shaped counter plate. The film count value is indicated by rotating this counter plate to a degree corresponding to one frame at a time. The film counter mechanism includes a detecting mechanism. The detecting mechanism consists of a cam and an operating piece and changes the state of a switch in response to the movement of the operating piece. The cam is provided with a cut-out part which corresponds to three frames. When a three-frame portion of the film is wound after commencement of the prewinding action, the operating piece swings by riding over the cam from the cut-out part to change the state of the switch. In the case of this patent, the failure of the automatic film loading is detected by operating the detecting mechanism in combination with an electric timer. More specifically, the camera is arranged to determine that an automatic film loading action has failed if the state of the switch does not change within a given period of time set at the electric timer from the start of the prewinding action after the back lid of the camera is closed. Upon detection of the failure of the film loading, the film driving motor is immediately brought to a stop.